For M-profile exception returns, the mmu index to use for exception
return unstacking is supposed to be that of wherever we are returning to:
 * if returning to handler mode, privileged
 * if returning to thread mode, privileged or unprivileged depending on
   CONTROL.nPRIV for the destination security state

We were passing the wrong thing as the 'priv' argument to
arm_v7m_mmu_idx_for_secstate_and_priv(). The effect was that guests
which programmed the MPU to behave differently for privileged and
unprivileged code could get spurious MemManage Unstack exceptions.
Reported-by: NAdithya Baglody <adithya.nagaraj.baglody@intel.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20180709124535.1116-1-peter.maydell@linaro.org